Sputter deposition is widely used for depositing thin films and enables the fabrication of a broad range of devices and components. Sputter deposition is particularly useful for the fabrication of thin-film devices for optical applications. Such thin-film devices can include a single layer or a plurality of layers ranging in number from two to several thousand. The spectral performance of such thin-film devices depends on the thicknesses of the layers they include. Thus, the capability to deposit layers to a reference deposition thickness is of critical importance.
A typical sputter-deposition system includes a target cathode, an anode, a substrate, a plasma, and a power supply. The target cathode, anode, and substrate are disposed within a vacuum chamber, into which a gas is introduced. The power supply, which is located outside the vacuum chamber, is used to apply a voltage between the target cathode and the anode, hereafter referred to as cathode voltage. The cathode voltage partially ionizes the gas in the vacuum chamber, creating a plasma. The plasma contains positively charged ions, which are attracted to the negatively charged target cathode and accelerate towards it. When the ions collide with the target cathode, target material is sputtered from the target cathode. The sputtered target material deposits as a layer on the substrate, as well as on other surfaces of the sputter-deposition system. Typical target materials include: metallic elements, such as tantalum, niobium, and aluminum; semiconducting elements, such as silicon and germanium; and conductive oxides, such as (In2O3)1-x(SnO2)x (indium tin oxide (ITO)), Ta2O5-x, and TiO2-x, among others.
Many variants of sputter deposition have been developed. In magnetron sputter deposition, a magnetron is included in the sputter-deposition system to produce magnetic fields in the vicinity of the target cathode. The magnetic fields confine electrons and create a denser plasma to increase the sputtering rate. In reactive sputter deposition, a gas mixture of an inert gas and a reactive gas is introduced into the vacuum chamber of the sputter-deposition system, and the layer is formed by chemical reaction between the target material and the reactive gas. In pulsed direct-current (DC) sputter deposition, the voltage between the target cathode and the anode is periodically reversed to a small opposite voltage to minimize charge build-up and arcing. In alternating-current (AC) sputter deposition, an AC voltage is applied between two target electrodes, which alternate being the cathode and the anode to minimize arcing. In radio-frequency (RF) sputter deposition, an RF voltage is applied between a target electrode and a second electrode to minimize charge build-up, allowing nonconductive materials to be sputtered. In high-power pulsed magnetron sputter deposition, short high-power pulses are supplied to the target cathode in a magnetron sputter-deposition system to produce a plasma so dense that a large proportion of the sputtered target material is ionized, leading to deposition of dense and well-adhering layers.
In any sputter-deposition technique, the sputtering rate of target material from the target cathode and the related deposition rate of the layer on the substrate are influenced by a large number of operating parameters, among them the power supplied to create and maintain the plasma, hereafter referred to as cathode power, the current drawn by the target cathode, hereafter referred to as cathode current, the impedance of the plasma between the target cathode and the anode, hereafter referred to as cathode impedance, and cathode voltage.
When the deposition rate of a layer is known, a deposition time at which the layer has a reference deposition thickness may be determined. The dependence relationship d(T) of the deposition thickness of the layer on time is equal to an integral over time of the dependence relationship r(t) of the deposition rate on time, according to:
                              d          ⁡                      (            T            )                          =                              ∫            0            T                    ⁢                                    r              ⁡                              (                t                )                                      ⁢                                          ⅆ                t                            .                                                          (        1        )            Typically, the power supply of the sputter-deposition system is configured to provide constant cathode power, and the deposition rate is assumed to be a constant reference deposition rate rr, such that:d(T)=rrT.  (2)In some instances, a constant offset term b may be included in Equation (2) to correct for any transient variation in the deposition rate over time, arising from starting or stopping deposition of the layer, or from delays between computer commands and device responses, for example, giving:d(T)=rrT+b.  (3)For simplicity, such a constant offset term will not be explicitly considered in the following.
Conventionally, a reference deposition time tr at which the layer has a reference deposition thickness dr is determined on the basis of the reference deposition rate rr, according to:
                              t          r                =                                            d              r                                      r              r                                .                                    (        4        )            However, this approach relies on the assumption that the deposition rate is constant when cathode power is held constant at a reference value, which is often incorrect.
For instance, during deposition of a layer at constant cathode power, the accumulation of sputtered target material on surfaces of the sputter-deposition system may lead to changes in cathode impedance. To maintain constant cathode power, the power supply automatically adjusts cathode voltage and cathode current to compensate for the changes in cathode impedance. The variation in cathode voltage and cathode current over time may result in departures of the deposition rate from the reference deposition rate over time and, thus, in errors in the deposition thickness of the layer.
To maintain a constant reference deposition rate of a layer, one or more operating parameters of the sputter-deposition system can be adjusted during deposition of the layer. For example, the deposition rate can be regulated by adjusting cathode power, as described in U.S. Pat. No. 4,166,783 to Turner, U.S. Pat. No. 5,174,875 to Hurwitt, et al., and U.S. Pat. No. 5,911,856 to Suzuki, et al., by adjusting a magnetic field during magnetron sputter deposition, as described in U.S. Pat. No. 4,500,408 to Boys, et al., by adjusting a flow rate of a reactive gas during reactive sputter deposition, as described in U.S. Pat. No. 5,911,856 to Suzuki, et al., U.S. Pat. No. 6,475,354 to Toyama, and World Patent Application No. WO 2006/032925 to Gibson, et al., or by adjusting plasma density, as described in U.S. Pat. No. 6,554,968 to Kearney, et al.
In particular, a method of regulating the deposition rate of a layer by adjusting the composition of a gas mixture during reactive sputter deposition is disclosed in U.S. Pat. No. 6,746,577 to Barber, et al. During deposition of the layer, cathode current or cathode voltage is held constant at a reference value, and the composition of the gas mixture is regulated to maintain cathode impedance at a reference value. Hence, a nearly constant reference deposition rate is maintained. To compensate for any momentary variation in the deposition rate over time, the energy delivered to the target cathode, hereafter referred to as cathode energy, is summed over time during deposition of the layer. Deposition of the layer is automatically stopped once a reference cathode energy has been supplied.
As an extension of the strategy of maintaining a constant reference deposition rate of a layer, the growth of the layer or the erosion of the target cathode may be directly monitored during deposition of the layer to ascertain the deposition rate of the layer. If departures of the deposition rate from the reference deposition rate are detected, operating parameters may be adjusted accordingly, as described in U.S. Pat. No. 5,754,297 to Nulman, U.S. Pat. No. 5,955,139 to Iturralde, and U.S. Pat. No. 7,324,865 to Sonderman, et al., for example. Similarly, a deposition rate of a layer ascertained by directly monitoring the growth of the layer or the erosion of the target cathode may be used to determine a deposition time at which the layer has a reference deposition thickness, as described in U. S. Patent Application No. 2006/0144335 to Lee, et al., for example.
Variants of such strategies have also been developed for improving the uniformity of the deposition thickness of a layer on particular substrates, such as stepped wafers, as disclosed in U.S. Pat. No. 4,957,605 to Hurwitt, et al., or lens elements, as disclosed in U.S. Pat. No. 6,440,280 to Burton, et al.
An object of the present invention is to overcome the shortcomings of the prior art by providing a simple method and control system for depositing a layer in a sputter-deposition system having a target cathode. Prior to deposition of the layer, a first dependence relationship of a deposition rate of the layer on an operating parameter selected from cathode voltage, cathode current, and cathode power is provided. During deposition of the layer, instead of stabilizing the deposition rate by adjusting the operating parameter, the operating parameter is allowed to drift over time. A second dependence relationship of the operating parameter on time is measured, while a different operating parameter, also selected from cathode voltage, cathode current, and cathode power, is held substantially constant. On the basis of the first and second dependence relationships, a deposition time for the layer is dynamically determined during deposition of the layer, without directly monitoring the growth of the layer or the erosion of the target cathode.